1. Field of Invention
The present invention provides a water purification device utilizing electrochemically generated hydroxyl free radicals.
2. Related Patent Applications
The electrodes and electrochemical cells utilized in this water purification device and a method for producing said electrodes are provided in the related US Patent Application titled "Electrode, Electrode Manufacturing Process, and Electrochemical Cell", Ser. No. 08/194,727, filed Feb. 10, 1994 by the same inventors. These electrodes include a titanium metal substrate with an oxide coating comprised of titanium dioxide that is doped with niobium or tantalum, wherein the mole fraction of niobium or tantalum in the +4 valence state relative to total metal is 0.25 percent or greater.
The above identified related patent application Ser. No. 08/194,727 is hereby incorporated by reference.
3. Discussion of Prior Art
Electrochemical methods are sometimes used to remove or decompose chemical impurities in water. For example, cathodic reduction is used to remove heavy metal ions including copper, nickel, and silver (Kuhn 1971). Anodic oxidation may be used to destroy cyanide and phenols (Kuhn 1971a), ammonia (Marincic and Leitz 1978), and organic dyes (Abdo and Al-Ameeri 1987). All of these applications involve very specific anodic reactions involving very specific substrates that occur at moderate anodic potentials far below the potential required to generate hydroxyl. Only partial oxidation of the target substrate is achieved. Undesirable byproducts may be formed; for example, electrolytic oxidation of phenol may produce some amount of chlorophenol, an even more objectionable water pollutant. No prior art recited above is able to completely oxidize organic substrates, nor oxidize in a nonspecific manner a wide variety of chemical substances dissolved in water.
An electrochemical method involving the generation of NO.sub.3. radicals in a medium containing nitric acid has been reported (Gedye and others 1987). The free radicals produced react with dissolved organic compounds and destroy them. This reaction requires a high concentration of nitric acid, and the electrolyte solution is highly corrosive. It is therefore practically limited to destroying organic compounds dissolved in strong acid solutions.
Hydroxyl free radical is a very powerful, nonspecific oxidizing species which attacks most organic molecules as well as oxidizable inorganic molecules and ions (Buxton and others, 1988). Hydroxyl free radical is produced by irradiation with ultraviolet light of particles of titanium dioxide dispersed in water (Kormann and others, 1991), and hydroxyl thus produced reacts with and degrades organic substances in solution. The hydroxyl radicals produced by the photochemical reaction are believed to be bound to the surface of the TiO.sub.2 particles (S.OH), and the oxidation of the substrate occurs at the surface (same ref.). The photochemical method remains largely a laboratory curiosity, because sunlight contains only a small fraction of usable UV energy, and the photochemical method has a small quantum yield. In the laboratory, hydroxyl is produced by reaction of hydrogen peroxide with iron salts dissolved in mildly acidic solution, called Fenton's Reaction.
CHEZ, U.S. Pat. 4,676,878 described the electrochemical production of hydroxyl free radical utilizing electrodes with various semiconducting surface compositions. Chez teaches that at least 12.6 volts must be applied to power the electrochemical cell, which he describes as consisting of a single anode, a single cathode, and aqueous electrolyte solution between them; that is, a unit cell. In reference to five of his seven Examples, Chez stated that 14.5 volts was applied to the unit cell, and in reference to two of his examples he stated that current density averaged 0.0025 amperes per square inch =0.4 mA cm.sup.-2. Chez did not report anode potential, but at this current density the cathode potential would have been between 0 and -2 volts; therefore, in Chez's Examples V.sub.cell =14.5 V corresponds to E.sub.anode =12.5-14.5 V vs. NHE, and the 12.6 V unit cell voltage that Chez teaches is necessary to produce hydroxyl free radical corresponds to E.sub.anode =10.6-12.6 V vs. NHE. Chez also teaches that the part of the surface of the anode whereupon hydroxyl free radicals are generated should be coated with a p-type semiconductor.
BIANCHI, U.S. Pat. Nos. 3,948,751 and 4,003,817 has described electrodes wherein a titanium metal base is covered with an oxide coating which contains titanium dioxide, a large proportion of a platinum group metal, and in some examples also niobium or tantalum. The electrode preparation methods described by Bianchi involve brushing on to a titanium metal substrate a solution which contains compounds of the metals desired in the oxide coating, and then heating the electrode in air to evaporate and thermally decompose the coating solution and produced the desired oxide coating.
The electrodes described by Bianchi cannot be operatively combined with Chez to produce hydroxyl free radicals. The platinum group metals in the oxide coating will catalyze the electrolysis of water to molecular oxygen at a value of anode potential much less than that required to produce hydroxyl free radical. If such an electrode is polarized to a potential large enough, in principle, to produce hydroxyl free radicals, it will produce abundant oxygen bubbles, but little or no hydroxyl free radical.
Bianchi also mentions as a ramification the possibility of producing an electrode wherein a titanium metal base is covered with an oxide coating consisting of titanium dioxide doped with either niobium or tantalum, but no platinum group metal. Bianchi apparently did not implement this possibility, because his examples do not include electrodes which correspond to this description. The reason this possibility was not implemented by Bianchi is that the resulting electrodes would be inoperative due to low electrical conductivity of the oxide coating resulting.
It is essential that at least part of the Nb or Ta in the oxide coating be in the +4 valence state, because the single valence electron remaining in Nb.sup.+4 or Ta.sup.+4 provides the n-type doping which imparts useful electrical conductivity to the oxide coating. In the electrodes disclosed in the above identified Related Patent Application hereby incorporated by reference, Nb or Ta in the oxide coating is converted to the +4 valence state by annealing the coated electrodes under hydrogen containing a small amount of water vapor, and this annealing process imparts useful conductivity to the electrodes.
NIDOLA, U.S. Pat. No. 4,110,180 describes electrodes comprising a Ti-metal substrate with an oxide coating that includes titanium dioxide and a platinum group metal oxide (ruthenium dioxide in Nidola's examples), wherein the Ti-metal substrate is alloyed with up to ten percent Nb or Ta. The presence of the platinum group metal oxide makes Nidola's electrodes inoperable for production of hydroxyl free radicals, and the Nb or Ta in the electrode is in the Ti-metal substrate, not in the oxide coating.
In WERES et al., U.S. Pat. No. 5,364,508 we have demonstrated that hydroxyl free radicals can be produced and organic substances dissolved in water can be oxidized at E.sub.anode less than four volts vs. NHE, a finding that is surprising and unexpected in light of Chez's teachings. We have further demonstrated that some organic compounds dissolved in water can be oxidized by surface bound hydroxyl free radicals at the surface of the anode at E.sub.anode less than two volts vs. NHE. We have demonstrated the operability of these methods at current densities much higher than disclosed by Chez. We have further found that operating our anodes in the range of anode potential taught by Chez damages them.
The possibility of generating hydroxyl free radical by the oxidation of water or hydroxide ion at the surface of an anode is not obvious, because most electrode materials are sufficiently electrocatalytic to oxidize water and generate oxygen at an electrode potential far below that required to generate hydroxyl. For example, conventional titanium anodes are doped with platinum group metals to catalyze the evolution of oxygen, thereby allowing them to operate at a potential not much greater than that required to generate molecular oxygen, which is equal to 1.19-0.0592 pH volts at room temperature. Doping with niobium or tantalum has no such effect. In addition, most electrode materials, including nearly all metallic compositions, will corrode severely when exposed to a positive potential large enough to generate hydroxyl free radical. Because of their composition, our anodes provide a good yield of hydroxyl free radicals and do not corrode when operated at a potential positive enough to generate hydroxyl free radical.